Abstract [en]

Human glutathione transferase A1-1 (GST A1-1) has a flexible C-terminal segment that forms a helix (alpha9) closing the active site upon binding of glutathione and a small electrophilic substrate such as 1-chloro-2,4-dinitrobenzene (CDNB). In the absence of active-site ligands, the C-terminal segment is not fixed in one position and is not detectable in the crystal structure. A key residue in the alpha9-helix is Phe 220, which can interact with both the enzyme-bound glutathione and the second substrate, and possibly guide the reactants into the transition state. Mutation of Phe 220 into Ala and Thr was shown to reduce the catalytic efficiency of GST A1-1. The mutation of an additional residue, Phe 222, caused further decrease in activity. The presence of a viscosogen in the reaction medium decreased the kinetic parameters k(cat) and k(cat)/K(m) for the conjugation of CDNB catalyzed by wild-type GST A1-1, in agreement with the view that product release is rate limiting for the substrate-saturated enzyme. The mutations cause a decrease of the viscosity dependence of both kinetic parameters, indicating that the motion of the alpha9-helix is linked to catalysis in wild-type GST A1-1. The isomerization reaction with the alternative substrate Delta(5)-androstene-3,17-dione (AD) is affected in a similar manner by the viscosogens. The transition state energy of the isomerization reaction, like that of the CDNB conjugation, is lowered by Phe 220 as indicated by the effects of the mutations on k(cat)/K(m). The results demonstrate that Phe 220 and Phe 222, in the dynamic C-terminal segment, influence rate-determining steps in the catalytic mechanism of both the substitution and the isomerization reactions.

Pettersson, Pär L.

Abstract [en]

The present work focuses on the glutathione transferase (GST) Alpha-class enzymes, their characteristics as steroid isomerases and structural plasticity as malleable scaffolds for protein design. The GSTs are a family of detoxication enzymes that appears to have a wider variety of additional functions.

Kinetic steady-state parameters for human GST A1-1 with the steroid isomerase substrate Δ5-androstene-3,17-dione (AD), an intermediate in steroid hormone biosynthesis, were determined. It was established that GST A1-1 is a highly efficient steroid isomerase with a 30-fold higher catalytic efficiency, in terms of kcat/Km, than 3β-hydroxysteroid dehydrogenase/Δ5→4-isomerase, the enzyme regarded as the mammalian Δ5→4-isomerase in steroid hormone biosynthesis. Kinetic parameters were also determined for GST A2-2, GST A4-4 and the GST A1-1 mutant Y9F. From the dependency on pH of the kinetic parameters it was established that efficient catalysis requires glutathione (GSH) in its deprotonated form and it is suggested that the GSH-thiolate acts as a base in the catalysis of the Δ5→4-3-ketosteroid isomerase reaction.

GST A2-2 is a poor catalyst of the steroid isomerase reaction while GST A3-3 is highly efficient. Their catalytic efficiencies (kcat/Km) differ 5000-fold. Stepwise point mutations were performed to GST A2-2 in order to insert the amino acid residues from the active-site of GST A3-3 that distinguishes the two isoenzymes. The result was that GST A2-2 was redesigned to a highly efficient double-bond isomerase with both the catalytic constant (kcat) and catalytic efficiency (kcat/Km) in the same order as for GST A3-3. Furthermore, this was done by only exchanging amino-acid residues with first-sphere interactions, providing empirical proof-of principle for knowledge-based enzyme design.

Kinetic studies on GST A1-1 and a T68E mutant of GST A1-1 were also performed with a GSH analog lacking the g-glutamate a-carboxylate (dGSH), and using three different electrophilic substrates (AD; 1-chloro-2,4-dinitrobenzene, CDNB; 4-nitrocinnamaldehyde). Deletion of the a-carboxylate from the GSH glutamate had a severe impact on all reaction constants and it changed the rate-limiting step for the CDNB reaction as well as changed the pKa value for the enzyme-bound GSH thiol. The loss in activity caused by dGSH could in part be compensated by the T68E mutant contributing an enzyme-bound carboxylate instead.

The C-terminus of GST A1-1 is flexible and folds over the active site when the enzyme binds a substrate. Phenylalanine residues in the C-terminal end, known to interact with active-site residues tyrosine 9 and phenylalanine 10, were mutated to abolish those interactions. Studies of viscosity dependence for CDNB and AD with regard to kcat and kcat/Km showed that the dynamic C-terminal segment influence rate-determining steps for both the larger isomerase substrate, AD, as well as for the smaller conjugation substrate, CDNB.

Place, publisher, year, edition, pages

Uppsala: Acta Universitatis Upsaliensis, 2002. p. 38

Series

Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1104-232X ; 719